The present invention is in the field of semiconductor wafer processing, and more specifically relates to a disposable polishing pad for use in chemical mechanical polishing. The polishing pad contains an optical sensor for monitoring the condition of the surface being polished while the polishing operation is taking place, thus permitting determination of the endpoint of the process.
In U.S. Pat. No. 5,893,796 issued Apr. 13, 1999 and in continuation U.S. Pat. No. 6,045,439 issued Apr. 4, 2000, Birang et al. show a number of designs for a window installed in a polishing pad. The wafer to be polished is on top of the polishing pad, and the polishing pad rests upon a rigid platen so that the polishing occurs on the lower surface of the wafer. That surface is monitored during the polishing process by an interferometer that is located below the rigid platen. The interferometer directs a laser beam upward, and in order for it to reach the lower surface of the wafer, it must pass through an aperture in the platen and then continue upward through the polishing pad. To prevent the accumulation of slurry above the aperture in the platen, a window is provided in the polishing pad. Regardless of how the window is formed, it is clear that the interferometer sensor is always located below the platen and is never located in the polishing pad.
In U.S. Pat. No. 5,949,927 issued Sep. 7, 1999 to Tang, there are described a number of techniques for monitoring polished surfaces during the polishing process. In one embodiment Tang refers to a fiber-optic ribbon embedded in a polishing pad. This ribbon is merely a conductor of light. The light source and the detector that do the sensing are located outside of the pad. Nowhere does Tang suggest including a light source and a detector inside the polishing pad. In some of Tang""s embodiments, fiber-optic decouplers are used to transfer the light in the optical fibers from a rotating component to a stationary component. In other embodiments, the optical signal is detected onboard a rotating component, and the resulting electrical signal is transferred to a stationary component through electrical slip rings. There is no suggestion in the Tang patent of transmitting the electrical signal to a stationary component by means of radio waves, acoustical waves, a modulated light beam, or by magnetic induction.
In another optical end-point sensing system, described in U.S. Pat. No. 5,081,796 issued Jan. 21, 1992 to Schultz there is described a method in which, after partial polishing, the wafer is moved to a position at which part of the wafer overhangs the edge of the platen. The wear on this overhanging part is measured by interferometry to determine whether the polishing process should be continued.
In earlier attempts to mount the sensor in the polishing pad, an aperture was formed in the polishing pad and the optical sensor was bonded into position within the aperture by means of an adhesive. However, subsequent tests revealed that the use of an adhesive could not be depended upon to prevent the polishing slurry, which may contain reactive chemicals, from entering the optical sensor and from penetrating through the polishing pad to the supporting table.
In conclusion, although several techniques are known in the art for monitoring the polished surface during the polishing process, none of these techniques is entirely satisfactory. The fiber optic bundles described by Tang are expensive and potentially fragile; and the use of an interferometer located below the platen, as used by Birang et al., requires making an aperture through the platen that supports the polishing pad. Accordingly, the present inventor set out to devise a monitoring system that would be economical and robust, taking advantage of recent advances in the miniaturization of certain components.
The disposable polishing pad described below is composed of foamed urethane. It contains an optical sensor for monitoring, in situ, an optical characteristic of a wafer surface being polished. The real-time data derived from the optical sensor enables, among other things, the end-point of the process to be determined without disengaging the wafer for off-line testing. This greatly increases the efficiency of the polishing process.
The wafers to be polished are composite structures that include strata of different materials. Typically, the outermost stratum is polished away until its interface with an underlying stratum has been reached. At that point it is said that the end point of the polishing operation has been reached. The polishing pad and accompanying optics and electronics is able to detect transitions from an oxide layer to a silicon layer as well as transitions from a metal to an oxide, or other material.
The polishing pad described involves modifying a conventional polishing pad by embedding within it an optical sensor and other components. The unmodified polishing pads are widely available commercially, and the Model IC 1000 made by the Rodel Company of Newark, N.J., is a typical unmodified pad. Pads manufactured by the Thomas West Company may also be used.
The optical sensor senses an optical characteristic of the surface that is being polished. Typically, the optical characteristic of the surface is its reflectivity. However, other optical characteristics of the surface can also be sensed, including its polarization, its absorptivity, and its photoluminescence (if any). Techniques for sensing these various characteristics are well known in the optical arts, and typically they involve little more than adding a polarizer or a spectral filter to the optical system. For this reason, in the following discussion the more general term xe2x80x9coptical characteristicxe2x80x9d is used.
In addition to the optics the disposable pad provides an apparatus for supplying electrical power to the optical sensor in the polishing pad.
The disposable polishing pad also provides an apparatus for supplying electrical power for use in transmitting an electrical signal representing the optical characteristic from the rotating polishing pad to an adjacent non-rotating receiver. The pad is removably connectable to a non-disposable hub that contains power and signal processing circuitry.
An optical sensor that includes a light source and a detector is disposed within a blind hole in the polishing pad so as to face the surface that is being polished. Light from the light source is reflected from the surface being polished and the detector detects the reflected light. The detector produces an electrical signal related to the intensity of the light reflected back onto the detector.
The electrical signal produced by the detector is conducted radially inward from the location of the detector to the central aperture of the polishing pad by a thin conductor concealed between the layers of the polishing pad.
The disposable polishing pad is removably connected, both mechanically and electrically, to a hub that rotates with the polishing pad. The hub contains electronic circuitry that is concerned with supplying power to the optical sensor and with transmitting the electrical signal produced by the detector to non-rotating parts of the system. Because of the expense of these electronic circuits, the hub is not considered to be disposable. After the polishing pad has been worn out from use, it is disposed of, along with the optical sensor and the thin conductor.
Electrical power for operating the electronic circuits within the hub and for powering the light source of the optical sensor may be provided by several techniques. In one embodiment, the secondary winding of a transformer is included within the rotating hub and a primary winding is located on an adjacent non-rotating part of the polishing machine. In another embodiment, a solar cell or photovoltaic array is mounted on the rotating hub and is illuminated by a light source mounted on a non-rotating portion of the machine. In another embodiment, electrical power is derived from a battery located within the hub. In yet another embodiment, electrical conductors in the rotating polishing pad or in the rotating hub pass through the magnetic fields of permanent magnets mounted on adjacent non-rotating portions of the polishing machine, to constitute a magneto.
The electrical signal representing an optical characteristic of the surface being polished is transmitted from the rotating hub to an adjacent stationary portion of the polishing machine by any of several techniques. In one embodiment, the electrical signal to be transmitted is used to frequency modulate a light beam that is received by a detector located on adjacent non-rotating structure. In other embodiments, the signal is transmitted by a radio link or an acoustical link. In yet another embodiment, the signal is applied to the primary winding of a transformer on the rotating hub and received by a secondary winding of the transformer located on an adjacent non-rotating portion of the polishing machine. This transformer may be the same transformer used for coupling electrical power into the hub, or it can be a different transformer.
There must be a viable optical path between the top of the sensor and the lower side of the wafer. However, a void would not be acceptable, because it would quickly become filled with polishing slurry, thereby rendering it incapable of serving as an optical medium. In addition, a void would present a large mechanical discontinuity in the otherwise homogenous and uniformly resilient polishing pad. Further, the components of the optical sensor must not come into direct mechanical contact with the wafer that is being polished, to avoid scratching the surface of the wafer.
To overcome this problem, the optical sensor is embedded into the polishing pad using techniques described in detail below. These techniques have been successful in overcoming the disadvantages described above.